The present invention relates to a process and to an installation for coating a metallic strip continuously with a coating layer, for example a zinc-based coating.
A process exists for coating a steel strip with a coating layer, according to which the coating is applied to the strip by means of a coating cylinder immersed in the coating bath. This coating cylinder is in contact with the face of the strip to be coated and is driven to rotate in the direction of travel of the strip. In certain installations which carry out this process the coating cylinder is wiped by another cylinder of smaller diameter which is arranged upstream of the point of contact of the strip with the coating cylinder and in other installations the coating cylinder is immersed in the coating bath by less than half its lateral surface. Yet other installations make use of a back-pressure roller which is arranged against the face of the strip opposite that which receives the coating layer. Whatever the installation used, the process with coating cylinder has hitherto enabled only relatively little coating material to be applied to the strip and has thus limited the range of coating thicknesses which can be obtained.
The applicant has found that this limitation of thickness results from the fact that the layer of coating material applied by the coating cylinder to the strip is, in fact, wiped by the coating cylinder itself because of its rotation in the direction of travel of the strip. Moreover, in addition to limiting the thickness of the coated layer, the coating cylinder risks marking the layer and, in so doing, impairing the uniformity of the layer when changes in strip widths are made. In reality, according to this known process the effect of the coating cylinder is twofold: on the one hand, it transfers the coating material onto the strip and, on the other hand, at the same time, it wipes the coating layer and limits the thickness of same.
The problem arising with the known process is therefore that the thickness of the coating layer remains limited.